An important approach that has emerged in the last few decades is the design of molecular (homogeneous) catalysts or reagents for the oxidative functionalization of hydrocarbons based on the C—H activation reaction. This involves reaction of a regenerable M-X catalyst or reagent (M being a main group element in an oxidized state and X being one or more charge-balancing counterions) with a C—H bond of a hydrocarbon (R—H) under relatively mild conditions to selectively generate an M-R intermediate that can be converted to the desired R—X product with regeneration of M-X (eq.1).

There has been significant effort in this area of research with homogeneous as well as heterogeneous catalysts, and substantial progress has been made in recent years. Most of the work on the homogeneous systems have been primarily based on transition metals (with unfilled d-shells, d<10), such as Pt, Pd, Rh, and Ir. In contrast, relatively few studies have been directed toward the classic main group elements with a filled d-shell (d10). In 1993, Periana reported an example of a main group, metal cation, HgII, in the superacid solvents, concentrated H2SO4 and CF3SO3H, for direct conversion of methane to methanol esters (Periana et al., Science 259, 340-343 (1993)); see also International Patent Application WO 92/14738). In spite of the simplicity of the HgII system, it was not further developed due to lack of reaction in more practical weaker acid media such as CF3CO2H (TFAH or HTFA), CH3CO2H (HOAc), or aqueous acids where product separation can be practical. Another key issue was that the reactions of ethane and propane were unselective with the HgII system.
TlIII was found to be active for methane oxidation to the ester. However, this activity was only examined in superacid media and only with methane. TlIII or PbIV systems in TFAH or with higher alkanes were not studied primarily due to the recognition that both TlIII (Eº=1.2 V) and PbIV (Eº=1.5 V) are stronger oxidants as well as electrophiles than HgII (Eº=0.9V). See, e.g., A. J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution. (International Union of Pure and Applied Chemistry, New York, N.Y., 1985). Consequently, on the basis of the general considerations at that time, these main group cations were understood to be more likely than HgII to initiate unselective radical reaction with the higher alkanes and to be inhibited by weaker, more nucleophilic acid solvents. This model of expected lack of reactivity of strong electrophilies in weak acid media also seemed consistent with the observation that in earlier work on Pt bipyrimidine complexes, the Pt(II) state was found to active in superacid media whereas the arguably more electrophilic Pt(IV) state was inactive.
There similarly is no direct process for the conversion of benzene to phenol. Phenol is typically made through indirect processes involving addition of propylene, chlorine, etc. A direct process has been challenging because of the low selectivity typically involved in the functionalization of an arene, such as benzene.
Thus, it would be desirable to provide a method to selectively and directly functionalize compounds, such as heteroalkanes and arenes, without the generation of significant amounts of by-products and waste and without the need for an expensive transition metal catalyst.